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9 ways to prevent mold from growing in your terrarium

When I think of mold, pictures of fuzzy white stuff growing on bread come to mind. Yuck. Even worse when you see it on your plants. So how can you proactively prevent mold from growing rather than waiting for a disaster to happen?

Terrarium mold can be prevented in several ways:

  1. Add springtails to your terrarium
  2. Avoid overwatering your terrarium
  3. Apply fungicide to your substrate
  4. Remove dead or decaying plant matter promptly from your terrarium
  5. Sterilize your substrate before adding it to your terrarium
  6. Give your terrarium more light
  7. Ensure adequate drainage beneath the substrate
  8. Air out your terrarium periodically
  9. Allow your terrarium to dry before watering

So now that you know what you can do to prevent terrarium mold, let’s get into why each method works.

How to stop mold from growing on plants

Mold is a type of fungus with over 100,000 different species. Some estimate that there are at least that many that have yet to be discovered.

Mold requires four things to grow:

  1. Presence of mold spores
  2. Mold food (decaying organic matter)
  3. Appropriate temperatures
  4. High humidity

On the top of that, there are three things that kill mold:

  1. Fungicide
  2. Mold consumers (springtails)
  3. UV light

If you look through the list at the top of this article, you’ll notice that all of the preventative measures either prevent mold from growing or kill mold outright. Let me walk you through how each one does that.

Add springtails to your terrarium to eliminate mold

Springtails are tiny white insects that terrarium lovers love to add to their terrariums (try saying that three times fast). You can think about them as tiny terrarium janitors.

Springtails love to feed on fungi and decaying organic matter. They are literally mold’s arch nemesis.

You can get a culture of these online and just add them to your terrarium. Or you can catch springtails yourself from your own backyard. Check out this article if you want to learn more about that.

Avoid overwatering your terrarium stops mold from growing

Excessive moisture is a major factor in mold growth. The higher the humidity, the greater risk of mold growth. 

If at all possible, avoid overwatering your terrarium. Not only does it promote mold growth, but it can also cause root rot which is a separate, but disastrous problem to deal with.

You’ll know that your terrarium is overwatered when you see heavy condensation 24/7 and a soaking wet substrate.

By reducing the humidity, you are creating an environment that is less favorable for mold growth. 

Apply fungicide to your substrate to kill mold

Applying fungus sign to your substrate is another way you can kill mold and mold spores outright and stop it from growing proactively.

There’s a number of different fungicides you can use including:

  • Hydrogen peroxide
  • Neem oil
  • Horticultural oil

If you are shopping around for a fungicide, always double check to make sure it’s suitable for use with plants. Anything labeled all natural is a good sign of that.

Remove decaying plant matter promptly from your terrarium to prevent mold

Since mold feeds on decaying plant matter, you can proactively remove any decaying plants parts before they have a chance to grow mold.

If mold has nothing to eat, it doesn’t have any way to grow.

Sterilize your substrate before adding it to your terrarium to stop mold from growing

Mold spores are everywhere, even where you can’t see them. Your substrate soil provides the perfect conditions for mold to grow. Your future mold problems could be sitting right under your nose without your knowledge, before you’ve even built your terrarium.

That’s why it can be helpful to sterilize your substrate before adding it to your terrarium. That’s going to kill off any mold laying dormant in your substrate before it has a chance to show itself.

To sterilize your substrate, bake it in an oven at 350 C for 20 minutes. The extreme heat will kill all living creatures in the soil, including any mold spores. If you want to be even more extreme, you can wipe down your container with rubbing alcohol to sterilize your container.

Give your terrarium more light to kill mold

Light is another factor that inhibits mold growth. The higher intensity of light you give to your terrarium, the lower your chances of getting mold growth. 

The reason this works: UV light scrambles mold spore’s DNA because they only have a very thin protective layer. They have no way to protect themselves from this level of radiation. On top of that, higher light levels will slightly decrease the humidity. That’s why you’ll mostly see mold growing in dark, shady areas.

This study (that looks like it was done by some undergrad college students) shows that the higher intensity of light used, the less mold will grow. Even a little bit of indirect indoor light does a little bit to reduce mold growth compared to total darkness.

But just be careful not to give your terrarium too much light. A closed terrarium acts like a mini greenhouse. Too much light can cause your terrarium’s temperature to rise too high for your plants. 

My recommendation is to place your terrarium near, but not directly under a windowsill, especially if it’s facing the south side of your home.

Ensure adequate drainage beneath the substrate to reduce humidity

Adequate drainage in your terrarium is going to help reduce the humidity, especially in your soil. 

Most terrariums should have a drainage layer to prevent water from stagnating in your soil. On top of that, you may also want to ensure your substrate has enough porosity to allow for adequate drainage. You don’t want to have a substrate that retains water and clumps together when wet.

To learn more about that, check out this detailed article on terrarium drainage layers.

Air out your terrarium periodically to clear out mold spores

Airing out your terrarium once in a while is going to help reduce humidity in your terrarium as well as clear out any mold spores lingering in your terrarium.

Good airflow is always a recommendation for homeowners looking to prevent mold. The principle is pretty much going to work the same for a terrarium.

Allow your terrarium to dry before watering to reduce humidity

This one should be pretty straightforward. By allowing your terrarium to dry between waterings, you are giving mold a smaller window of time to grow.

Don’t worry too much about underwatering your plants. As long as the soil isn’t bone dry, your terrarium should be fine. Plants generally will tolerate a little bit of underwatering better than overwatering.